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What Is a SIMM?

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  • Written By: K. Powell
  • Edited By: Lindsay D.
  • Last Modified Date: 18 March 2014
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SIMM, which stands for single in line memory module, is a type of computer memory used in the x86 and Pentium family of computers, as well as selected laser printers. A SIMM consists of a small circuit board that holds from two to greater than 32 random access memory (RAM) chips and plugs into sockets that run an 8-bit or 32-bit bus. There are two distinct types: a 30-pin module for older x86 computers and a 72-pin module common in 486 and Pentium systems. Modules may contain parity or no parity.

The first SIMM module was created in 1983 by Wang Laboratories’ James Clayton and used exclusively in IBM PS/2 computers. Soon afterward, SIMMs became the standard for memory modules, replacing socketed DIP and SIPP memory chips which took up considerably more space on the motherboard. SIMMs were subsequently replaced by the dual in line memory module, or DIMM.

Of the two types of SIMMs, the older 30-pin module range in size from 256 kilobytes up to 16 Megabytes (MB). Each module provides for one byte, or 8 bits, plus one additional bit if parity chip is present, of data. In contrast, the 72-pin module can come in sizes from 1MB up to 2,048MB and provide four bytes, or 32 bits of data at a time. Two notches, one at the end and another in the middle, help guide the SIMM into the right location on the socket. Single-sided and double-sided configurations are available.

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One of the primary differences between SIMMs and DIMMs lies in the location of the pins, or electrical contacts. The contacts along the front edge of a SIMM module are repeated again in the back. The contacts on DIMM modules differ for each side. Data path size is another notable difference between DIMMs and SIMMs. DIMMs use a 64-bit data path while SIMMs are limited to 32-bits. Finally, SIMMs must be installed in pairs to work correctly whereas only one DIMM module is required at a time.

Installing SIMMs as part of a computer memory upgrade is a fairly easy process. Always refer back to the technical manuals before performing any upgrades to verify the memory type.

First, turn off and unplug the computer. Remove the case and look for the memory slots on the motherboard. Dissipate extra static electricity by touching a metal object. Then, holding the memory module by the edges to avoid direct contact with the pins, insert the module into an empty socket at a 60° angle, and rotate it into place until the module is perpendicular to the motherboard. Replace the cover of the computer, reconnect the cables, and turn on the computer. In most cases, the system should recognize the new memory.

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